Zero-feedback, collaborative beamforming for emergency radio: Asymptotic analysis

Collaborative beamforming among a set of distributed terminals is studied, assuming a) no specialized RF hardware for carrier frequency synchronization, and b) zero feedback from destination (either in the form of pilot signals or explicit messages). Our goal is to provide a solution for conventional radios (not necessarily wideband), when the link between a single source transmitter and destination is too weak, so that no signal can be reliably received at the destination. In such critical case, zero feedback messages from destination to the multiple transmitters cannot be assumed, even when the destination is equipped with powerful hardware. A solution is provided for conventional radios in relevant critical applications, such as in emergency radio. The proposed scheme simply exploits lack of synchronization among distributed carriers, operating at the same nominal carrier frequency. It is shown that such beamforming is possible and its performance is analytically quantified. Results include asymptotic analysis for the case of large number of transmitters.